Generalized stability criterion for columnar to equiaxed grain transition during solidification upon vertical twin roll casting

To promote the columnar to equiaxed grain transition (CET) to obtain uniform equiaxed grain, most of the available theoretical works mainly focus on the growth but ignore the nucleation. Regarding it, a systematical framework integrating nucleation catalyzed by refiners with dendritic growth, was constructed, where, a new conception of generalized stability (GS), considering evolving thermodynamic driving force (ΔG) and kinetic energy barrier (Q) upon solidification of VTC, was adopted to study the CET position, the grain radius and the mechanical properties. By adding the refiners, especially the Al–5Ti–1B refiner, the larger thickness of 5–6 mm for equiaxed grain and finer grain radius of 19 μm are exhibited by increasing the ΔG for nucleation and the GS for growth, possessing simultaneously improved strength and plasticity. Following a criterion of high ΔG–high GS, the refiner diameter is optimized to be 0.3 μm.